The present invention is based on an elastic expansion appliance having the features indicated in the preamble of claim 1. An elastic expansion appliance of this kind is known from U.S. Pat. No. 5,167,500. The known elastic expansion appliance comprises two members joined to one another by two parallel guide rods, and a pseudoelastic coiled spring which is arranged between the two members. Each of the two guide rods is fastened at one of its ends in an orifice of one of the two members, and sits at its other end in a pertinent orifice of the opposing member, so that the two members can change the spacing between them by displacement along the two guide rods. The displacement can be effected by the coiled spring. The two members are intended either to be embedded in a plastic palate plate and/or to carry soldered-on or welded-on arms which respectively carry retention elements, clamps, or eyes which engage on teeth whose position is to be corrected, this occurring under the action of the spring. For this purpose, the elastic expansion appliance is installed into the dentition with the spring under stress, e.g. in the compressed state, and thereafter acts with the return force of the spring on the tooth or teeth whose position is to be corrected, the elastic expansion appliance being capable of expanding until the return force of the spring has declined to the point that it is no longer sufficient to correct tooth position. The magnitude of the correction which actually occurs thus results from the expansion which the spring performs until it has relaxed. If it is desired to change the magnitude of the correction, it is necessary either to change the distance ever which the elastic expansion appliance is initially compressed, and/or to replace the spring. Both are laborious and unsuitable for practical use. A further disadvantage is the fact that the preselected displacement travel is too great for the intended correction of the tooth position, which may become evident only in the course of the treatment. The physician performing the treatment can only ascertain this. However, if the patient comes in regularity for examination, it the patient does not do so, or does so ton late, the tooth position may already have been changed to such an extent that a countercorrection is necessary. This can be eliminated, however, by way of a jackscrew which is already disclosed in U.S. Pat. No. 5,167,500 as existing art and which displaces the two variable-spacing members not by way of a spring, but by way of a double spindle having sections threaded in opposite directions. In ardor to prevent excessive correction forces, a jackscrew of this kind can always be expanded by no more than 0.25 mm. For a typical tooth correction with 4 mm of tooth movement, the patient must therefore visit the doctor 16 times in order to have the jackscrew readjusted, which is laborious and tiresome. The came applies to the combined tension and compression screws according to Heller for sagittal movement of teeth.
An elastic jackscrew according to Hauser, disclosed in DE-824 632, allows for somewhat fewer readjustments than with a springless jackscrew. An elastic jackscrew of this kind is, however, more complex to manufacture than a springless (rigid) jackscrew.
It is the object of the present invention to indicate a way in which the progress of a tooth position correction can easily and reliably be kept under control.
This object is achieved by an elastic expansion appliance having the features indicated in claim 1. Advantageous developments of the invention are the subject of the dependent claims.
According to the present invention, in order to adjust the distance over which the spacing of the two members of the elastic expansion appliance can be changed, a stop of variable length is provided for the spring and/or for one of the two members, the length of the stop being measured in the direction of the linear guidance means. By way of such a stop, the distance over which the spring can relax, and thus the distance over which the spacing of the two members can change, can be changeably predefined. In this fashion, the physician performing the treatment can specifically define, by visual inspection, how much tooth correction is to occur until the first examination. There are no deleterious consequences even if the patient does not arrive for an agreed-upon examination appointment, since correction of the tooth position automatically ends when the preset distance has been reached.
In an advantageous development of the invention, the length of the stop can be changed only in steps. This has the advantage that the length of the steps can be selected so that on the one hand sufficient treatment progress is attained, and on the other hand the step is not so large that an excessive, incorrect change in tooth position might occur. Misadjustments can be prevented by suitably selecting the step length. In the simplest case, the length of the stops can be changed in equal steps. It is also possible, however to conFig. the stops so that they can be changed in steps of unequal length. This has the particular advantage that longer steps can be set at the beginning of the treatment, while as the end of the treatment approaches, a shorter length can be selected for the steps; as a result, the treatment duration and the number of examinations can he decreased.
In an advantageous development of the invention, in order to constitute a stop that can be changed only in steps, provision is made for a stack of spacers, in particular plate-shaped, which are provided between the spring and one of the members, or between one of the members and an immovable stop. The spacers con easily be placed onto the linear guidance means which advantageously are guide rods, optimally perpendicular to the longitudinal direction of the linear guidance means, since they can then be removed without disassembling the elastic expansion appliance or destroying the spacers. In order to allow the spacers to be placed onto the linear guidance means perpendicular to their longitudinal direction, they advantageously have a laterally open recess into which the linear guidance means engage. This laterally open recess can be a slot into which the linear guidance means are inserted, one after another, when the spacers are slid onto the two linear guidance means in the direction of the plane spanned by said means. The spacers can also, however, be slid into the linear guidance means perpendicular to the plane spanned by said means, if the spacers have two slots provided at the spacing of the linear guidance means. Instead, however, it would also be possible to provide half-round recesses at opposite ends of the plate-shaped spacers, and clamp them with the recesses only between the linear guidance means. To ensure that they are securely held on the linear guidance means, they are advantageously snapped on, which is easily possible due to the inherent elasticity of a plate-shaped and, in particular, slotted spacer.
If the spacers are arranged between the spring and one of the members, then as the thickness of the spacer stank increases, given a predefined initial spacing between the two members, the stress on the spring becomes greater, and the distance over which the elastic expansion appliance expands in the course of the treatment is greater. If, on the other hand, the spacer stack is arranged riot between the spring and one of the members but between the member and an outer immovable stop, then as the thickness of the spacer packet increases the spring is also compressed more strongly, but at the same time the possible displacement travel is also limited. In this case, the elastic expansion appliance is used in such a way that at the beginning of the tooth correction the two members have a smaller spacing than would he determined by the spacers being used, i.e. at the beginning of the treatment, the member adjacent to the spacers is not in contact against the spacer stack, but rather is brought closer to the other member against the spring force, so that the possible expansion distance is determined by the spacing between the member adjacent to the spacers and the spacer stack. Tooth correction comes to a halt when the aforesaid member comes into contact against the spacer stack. The change in tooth position achieved up to that point can then be checked by the physician performing the treatment, and the treatment can easily be continued by removing one of the spacers from the spacer stack and thereby enabling a further expansion step, at the end of which the progress of the treatment can once again be checked. This procedure just described is particularly advantageous, which is why it is preferred to arrange the variable-length stop not between the spring and one of the members, but rather between the member and an outer immovable stop. With the configuration of the elastic expansion appliance, the physician performing the treatment has a choice between different procedures. For example, he or she might at the outset remove all the spacers, and thereby allow from the outset the maximum displacement travel of the elastic expansion appliance. He or she could also, however, always remove only one of the spacers, in order to check the tooth correction status after each step. He or she could also initially remove two or three spacers so as to begin with a large correction step and then complete the correction in shorter individual steps. He or she even has the possibility of leaving it to the patient, according to a predefined schedule, to remove individual spacers which for this purpose, especially if they are of different thicknesses are marked in distinguishable fashion, e.g. by being differently colored. The removal of spacers is extremely easy for the patient: to do so, he or she can release the apparatus, of which the elastic expansion appliance is a constituent, from the dentition by gently compressing the elastic expansion appliance, and remove it from his or her mouth. He or she can then, in accordance with instructions provided by the physician performing the treatment, remove a spacer and their reinsert the apparatus back into the dentition in the same fashion. Misadjustments can be practically ruled out, since the treatment steps are predefined by the thickness of the spacer.
The elastic expansion appliance is particularly advantageous if what is used as the spring, which advantageously is conFIG.d as a coiled spring, is not a conventional steel spring but rather a spring made of a shape-memory alloy which is pseudoelastic at the temperatures prevailing in the mouth. Preferred shape-memory alloys are alloys based on nickel and titanium, which contain nickel and titanium in approximately equal atomic percentages. Alloys of this kind can exist, depending on the temperature selected, in either the austenitic or martensitic state. Martensite is present at lower temperatures, austenite at higher temperature. The temperature at which the alloy begins to convert from austenite to martensite during cooling is called the Ms point. In the martensitic state below the Ms point, alloys of this kind can exhibit shape memory: a plastic deformation that has occurred in the martensitic state can be reversed by heating to a temperature above the Ms point, in a temperature range just above the Ms point, a shape-memory alloy of this kind can exhibit pseudoelastic behavior. This pseudoelastic behavior, sometimes also called xe2x80x9csuperelasticity,xe2x80x9d is characterized by the tact that the force required for increasing elongation of the material initially rises sharply as in the case of an austenite, but then, after reaching en elongation of approximately 1% to 2%, increases only slightly as elongation progresses further, and does not increase steeply again until reaching greater elongations of 6% to 8%. The intermediate elongation region is called the xe2x80x9cmartensite plateau,xe2x80x9d the name deriving from the fact that martensite forms in the alloy in response to the tensile stress. When the tension on the materiel is released, it reverts to the austenitic state. These pseudoelastic elongations are highly reversible up to elongations of more than 6% to 8%. Because of the pronounced martensite plateau, pseudoelasticity does not obey Hooke""s law. Spring which exhibit this kind of pseudoelastic behavior are thus particularly suitable for the purposes of the present invention, since for spring travels in the region of the martensite plateau, the return force of the spring is almost independent of spring travel.
An elastic expansion appliance according to the invention thus has the great advantage that the pressure or tension exerted by the spring or the tooth remains almost unchanged for the duration of the treatment. Because the pressure or tension remains constant, the tooth position is changed more quickly then before. Since there is almost no change in spring force within the martensite plateau, it is moreover possible, by applying the invention, to reliably prevent excessive forces from inadvertently being set. The use of a pseudoelastic spring additionally makes it possible to define the step lengths by which the stop can be changed solely on the basis of the intervals at which an examination is desirable. There is no restriction on the step length for technical reasons, in contrast to elastic jackscrews with ordinarily elastic springs, in which the force exerted on the tooth increases in proportion to the step length and must be limited in order to prevent deleterious pressure spikes, and very much in contrast to jackscrews without springs, which in order to prevent deleterious pressure spikes should not be adjusted by more than 0.25 mm per step.
In the case of elastic expansion appliances according in the present invention having pseudoelastic springs, the selected step length by which the stop length can be modified is preferably between 0.75 mm and 1.25 mm, in particular 1 mm. In the majority of treatment instances, it is then possible to use only four treatment steps, end no more than four physician examinations. In contrast to the existing art with springless jackscrews which requite approximately four times as many treatment steps and physician examinations.
In the case of the elastic expansion according to the present invention, the spring can be arranged between the two members of the elastic expansion appliance in such a way that it attempts to increase the spacing between the two members. It is advantageous that the spring can, however, also be arranged so that it attempts to decrease the spacing between the two members. In this case the elastic expansion appliance experiences a negative expansion as the treatment progresses. Particularly suitable therefor is an arrangement of the spring in which it is retained in the one member between a stop provided immovably on that member and a stop that is displaceable in that member, the immovable stop being located closer to the other member than the displaceable stop, and the displaceable stop being joined to the other member via a tie rod. With this arrangement, the spring, initially loaded by compression, pulls the other member by way of the tie rod toward the one member as the tooth treatment progresses. In this context, the spring is preferably arranged so that it surrounds the tie rod, thus achieving a particularly compact configuration for the elastic expansion apparatus. A compact design is also promoted by a development of the invention according to which the spring is located in a cage which is mounted immovably on one member and projects beyond that one member toward the other member, which has a recess into which the projecting part of the cage can be introduced as the members approach one another. This cage, which is preferably a cylindrical sleeve, has the further advantage that it shields the spring and counteracts any deposition of food residues on the spring.
In another embodiment of the invention, there are provided in the elastic expansion appliance, instead of plate-shaped spacers, rod-shaped spacers which are arranged between linear guidance means in one of the members and extend perpendicular to the linear guidance means, so that they can he slid out of the elastic expansion appliance in their longitudinal direction. This embodiment also allows a very compact, space-saving design for the elastic expansion appliance, especially if the rod-shaped spacers do not project beyond the member in which they are arranged.
The rod-shaped spacers are intended, in this context, to rest against one another at their enveloping surfaces. For this purpose, they can be conFIG.d as rods with a rectangular cross section; in this case they can be guided against one another with their flat enveloping surfaces at which they touch one another. The rod-shaped spacers can also, however, be cylindrical. In order to be able to constitute a stable stack with cylindrical spacers, it is recommended that particular guidance means and/or holding means be provided in the member in which the cylindrical rods are arranged. For this purpose, the rod-shaped spacers in the one member are preferably arranged in a recess in which the guidance means and/or holding means which hold the spacers in their selected orientation are provided. These guidance means and/or holding means can be the wall which delimits the recess. It is also possible, however, to provide in the recess separate guidance means and/or holding means which perform this task alone or in combination with the wall of the recess. It is particularly advantageous to provide in the recess a shackle with which the spacers are held and guided linearly in the recess; this shackle is displaceable in the one member, and fastened in the other member. The shackle pulls the spacers, in response to one or more, preferably two springs, toward the other member of the elastic expansion appliance. When one of the spacers is removed, the spacing between the two members can be increased by the thickness of the rod-shaped spacer.
For good guidance and retention of the rod-shaped spacers in the recess of the one member, the shackle is preferably U-shaped, the two ends of the shackle being fastened in the other member of the elastic expansion appliance.
In order to be able to hold the rod-shaped spacers in the shackle, the spacers are preferably equipped with a constriction at which they snap in between the two arms of the shackle. The shackle is advantageously arranged in the recess with some lateral clearance, so that it can be elastically widened. This makes it easier, simply via pressure in their longitudinal direction, to snap the rod shaped spacers into piece and then release them from the snapped in position and slide them out of the elastic expansion appliance.
Preferably the recess is sealed except for two holes which are required for passage of the ends of the shackle, and with the exception of two aligned holes which are located above that rod-shaped spacer which is located in each case at the end wall of the recess which is adjacent to the other member of the elastic expansion appliance. Only the particular rod-shaped spacer located at this point can then he pushed nut of the elastic expansion appliance. As a result of the expansion of the spring(s) which thereupon takes place, the next rod-shaped spacer is pushed into the axis of these two aligned holes. The spacer is thereby largely protected from food residues in the recess; if necessary, the holes above and below it can be closed off with stoppers.
A plastic is preferably used as the material for the adjustable-length stop, which is conFIG.d in particular as a plate stack or a row of rods.
The invention is not limited to changing the stop only in steps. According to another advantageous development, the stop is made of a material which dissolves or decomposes in the mouth, so that the length of the stop automatically becomes shorter in the mouth, and a continuous adaptation of the displacement travel of the elastic expansion appliance thus takes place as the treatment progress. Suitable materials for a stop of this kind are polyglycolides (PGA), polyactides (PLA), stereocopolymers of PLA, copolymers of PLA, and resorbable ceramic materials. Also possible are hyaluronic acid and mixtures of the aforesaid materials, as well as polydioxanone, polyhydroxybutyric acid, copolymers of polyhydroxybutyric acid and hydroxyvaleric acid, as well as succinic acid polyesters.
In order for gradually dissolving or decomposing stop of this kind to shorten in defined fashion, it is advantageously located in a sheath which is open only at one or both ends but not along its outside diameter, so that decomposition or dissolution of the material can occur only in a manner proceeding from one or both ends, but not from all sides. It is possible by selecting the length of the stop to define the maximum displacement travel, and by way of the type of materiel to define the rate at which it dissolves or decomposes.